A series of experiments is proposed for investigating the nature of the basic capacities underlying speech perception in infants and the subsequent development of these capacities in adults. Data concerning the infant's discrimination and categorization of speech and speech-like acoustic stimuli will be collected using the high amplitude sucking procedure and operant headturning procedures. The information gained from these investigations with infants provides an index of the nature of the basic auditory capacities underlying speech perception at a point prior to the acquisition of a specific language. Research with both older children and adults using both measures of categorizaton and of discrimination serves to indicate the way in which such capacities develop as a result of acquiring and using a particular language. The proposed research, then, aims both to identify the basic auditory capacities underlying speech perception and to define the set of relevant experiences which shape the way in which adults process speech signals. This research has practical relevance for those studying communications disorders. Information about the way in which speech perception develops provides a baseline for assessing abnormalities in processing speech. Early identification of the development of such abnormalities is essential in formulating effective treatment programs.